Agricultural machines such as high-clearance sprayers are getting larger and more complex, with some sprayers having four-wheel steering to improve maneuverability, such as when making tight turns in a field's headlands. Such four-wheel steering systems typically have a controller that electronically controls a single steering valve which meters hydraulic fluid to both hydraulic rear-steering cylinders to correspond to a front-steering input. However, collision-induced rear wheel misalignment and/or steering errors at one rear wheel can be difficult for the controller to manage. Rear wheel misalignment or steering error can be caused, for example, by hitting a rock or other object which, in turn, can cause the rear wheels to lose sync and/or alignment and steer by incorrect amounts according to a steering algorithm. This can result in inaccurate four-wheel steering, poor handling characteristics and/or excessive wear on tires and other components. A need therefore exists for an improved system which eliminates one or more of the foregoing disadvantages.